The present invention relates generally to processes for improving zero dislocation yield and throughput of silicon single crystals grown in crucibles by the Czochralski method. The invention particularly relates to processes for preparing fused quartz crucibles having one or more surfaces which have been treated with a devitrification promoter.
Single crystal silicon which is the starting material for most processes for the fabrication of semiconductor electronic components is commonly prepared with the so-called Czochralski process. In this process, polycrystalline silicon ("polysilicon") is charged to a crucible, the polysilicon is melted, a seed crystal is immersed into the molten silicon and a single crystal silicon ingot is grown by slow extraction.
The crucible of choice for use in the Czochralski process is commonly referred to as a fused quartz crucible or simply a quartz crucible and is composed of an amorphous form of silica known as vitreous silica. One disadvantage associated with the use of vitreous silica, however, is the fact that contaminants on the inner surface of the crucible can nucleate and promote the formation of cristobalite islands in the vitreous silica surface (the islands being centered, in general, about the contamination site) as the polysilicon is melted and the single crystal ingot is grown. The cristobalite islands can be undercut and released as particles into the silicon melt, causing the formation of dislocations in the silicon ingot. The cristobalite islands can be undercut, for example, by the action of a low melting eutectic liquid formed at the interface between the vitreous silica and cristobalite as described by Liu et al., "Reaction Between Liquid Silicon and Vitreous Silica," J. Mater. Res., 7(2), p. 352 (1992). Other mechanisms by which the cristobalite islands are undercut and released into the melt are also known in the art.
Crucibles formed from vitreous silica may also exhibit a loss of structural integrity when subjected to the extreme temperatures experienced during the melting of the polysilicon charge or the growth of the silicon ingot. In general, these crucibles soften with increasing temperature and are soft enough to easily flow under an applied stress when the crucible wall temperature exceeds 1817.degree. K. Thus, graphite susceptors are frequently used to support the crucibles. Despite such reinforcement, however, quartz crucibles may buckle during the polysilicon melting and the crystal growth phases of the process, or when mechanical failure of the crystal puller occurs resulting in prolonged holding periods at high temperatures. Buckling occurs most often during remelt of an imperfect crystal or melting of bead polysilicon (i.e., granular polysilicon formed in a fluidized bed).
Pastor et al. disclose in U.S. Pat. No. 4,429,009 a process for converting the vitreous silica surface of a crucible to cristobalite for the purpose of passivating and enhancing the stability of the surface. In this process, the vitreous silica surface is exposed to an atmosphere containing atomic iodine at a temperature of 1200.degree. C. to 1400.degree. C. for about 24 hours to convert the surface to .beta.-cristobalite and then cooled to a temperature of less than 260.degree. C. which causes the .beta.-cristobalite to be transformed to .alpha.-cristobalite. When the crucible is thereafter reheated to an elevated temperature for use in a crystal growing process, the .alpha.-cristobalite layer transforms to .beta.-cristobalite. Experience has shown, however, that the .alpha.-cristobalite to .beta.-cristobalite phase transformations cause the devitrified surface to crack and form particulates on the surface. These particulates are released from the devitrified surface into the silicon melt, causing the formation of dislocations in the silicon ingot.
Other methods of treating crucible surfaces have also been proposed. Japanese Kokai No. 52/038873 discloses the use of a xenon lamp to irradiate the inner crucible surface in order to remove electrostatically adhering metallic contaminants to reduce formation of oxidation induced stacking faults in a silicon single crystal. Japanese Kokai No. 60/137892 describes a method of subjecting a crucible to electrolysis to remove alkali metals from the crucible which serves to reduce the incidence of lattice defects and crucible deformation. U.S. Pat. Nos. 4,956,208 and 4,935,046 describe crucibles having an opaque outer shell and an inner transparent quartz layer substantially free from bubbles for controlling the transfer of oxygen into a silicon melt. The inner layer is also described as being effective in suppressing the growth of cristobalite at the crucible melt interface, preventing the cristobalite from dropping off into the melt and disturbing growth of the crystal. Many of these treatments do not strengthen the walls of the crucible against deformation when subjected to severe temperatures, nor do they control the devitrification process in the presence of molten silicon.
U.S. Pat. No. 4,102,666 describes the formation of a thin crystalline silica layer on the outer surface of a diffusion tube to improve its thermal dimensional stability. The outer surface of the tube is treated with crystallization promoting nuclei such as oxides, carbides or nitrides of boron, aluminum, phosphorus, antimony, zinc, magnesium, calcium, gallium or Group IV elements of the periodic table. The nuclei promote very slow devitrification which is said to increase the useful life of the diffusion tube. The diffusion tubes are used in processing semiconductor wafers at temperatures of up to about 1300.degree. C. which is significantly below the softening point of vitreous silica.
There is a need for quartz crucibles having greater structural stability to prolong the useful life of the crucibles and prevent deformation and buckling of the crucibles during melt down and crystal growth. Crucibles that release fewer particulate contaminants into the silicon melt are also needed to improve the yield and throughput of zero dislocation (i.e., dislocation-free) single crystals grown by the Czochralski process.